This invention relates to analog signal processing devices, and more particularly to electronic filtering and mixing devices. Filtering devices receive electronic input signals containing a plurality of frequencies, including a desired channel (or frequency spectrum); and pass only signals within the desired channel. Mixing devices receive input signals and generate output signals having frequency spectrums proportional to the spectrum of the input signal and centered about new frequencies.
The invention herein described is called a programmable frequency converting filter because it performs both filtering and mixing operations; and in addition, the width of the channel that is passed is not fixed but is logically selectable. As an example, at one time instant, the programmable frequency converting filter may pass a band of frequencies corresponding to one single side band channel, and center this channel about new frequencies. Alternatively, at another time instant, the programmable frequency converting filter may pass a band of frequencies corresponding to one double side band channel, and center this channel about other new frequencies.
Applications for the programmable frequency converting filter are very broad. Typical uses include the processing of signals in radio receivers, television receivers, and CB transceivers. In the past, these devices required on circuit to filter single sideband channels and another circuit to filter double sideband channels. In comparison, the present invention selectively filters single sideband channels or double sideband channels with only one circuit.
Also, in the past, the filtering operation was performed by electronic circuits which were both expensive and too large to integrate by a semiconductor chip. This was because the narrow spacing between adjacent channels in the frequency spectrum requires the filter to have an abrupt transition from its passband to its stopband. The width of this transition is generally referred to as the skirt response of the filter. In order to achieve a narrow skirt response either multipole mechanical filters, lumped inductor-capacitor elements, or multipole crystal filters were required.
In comparison, the present invention utilizes a charge transfer device configured as a transversal filter to achieve the sharp skirt response. The charge transfer device may be a charge coupled device (CCD) or a bucket brigade device (BBD) as an example. The charge transfer device is both inexpensive and capable of being integrated on a single semiconductor chip.
In addition, the mixing operation or frequency converting operation was performed in the past by an oscillator circuit and a mixer circuit which were separate from the filter circuit. In comparison, the present invention performs frequency conversion by a charge injection sampling device which is an integral part of the tranversal filter. By taking periodic samples of the input signals, the sampling device creates new signals having frequency spectrums proportional to the input signals and being spaced about multiples of the sampling frequency. These new signals are then passed through the filter portion of the charge transfer device. The filter portion has multiple passbands each of which is proportional to the sampling frequency and centered about multiples of the sampling frequency. Thus, it passes the desired channel at multiples of the sampling frequency. A continuous filter having a single passband couples to the output of the transversal filter. This single passband is aligned with one of the multiple passbands of the transversal filter.
Still another important aspect of this invention is that the skirt response of the continuous filter need not be abrupt. This is because the sampling frequency of the transversal filter is high, and thus, the spacing between its multiple passbands is large. Therefore, the continuous filter doesn't require multipole mechanical elements or crystals, and is capable of being integrated on a semiconductor chip along with the transversal filter.
Accordingly, it is one object of the invention to provide an improved device for filtering electronic signals and for frequency shifting the filtered channel.
It is another object of the invention to provide a frequency converting filter having passbands of a programmable width.
Another object of the invention is to provide a programmable frequency converting filter having an abrupt transition from stopband to passband.
Still another object of the invention is to provide a programmable frequency converting filter that is capable of being integrated on a single semiconductor chip.